1. Field of the Invention
The present invention relates to a technology of an optical switch.
2. Description of the Related Art
Conventionally, a primary object of wavelength division multiplexing (WDM) in an optical transmission system is to achieve higher transmission capacity by increasing the number of channels. Recently, higher added value and lower operational costs of optical transmission systems have been expected from increased service menus using wavelength differences, and flexible band utilization.
However, conventional methods involving optical-to-electronic conversion of an optical signal followed by electronic switching and electronic-to-optical conversion, do not effect higher transmission capacity and lower operational costs. Hence, there is a need for a device that includes plural optical input and output ports, and that selectively manipulates an input multiplexed optical signal without electronic conversion. Included among such devices is an optical switch that, according to wavelength, selectively outputs optical signals included in the multiplexed optical-signal to different output ports respectively corresponding to the wavelengths, (see, for example, Marom, Dan M., et. al. “Effect of Mirror Curvature in MEMS Micro-mirror Based wavelength-selective Switches”).
FIG. 17 is a schematic of a conventional optical switch. A conventional optical switch 1700 includes a port unit 1710, a collimating lens unit 1720, a lens system 1730, a dispersing element 1740, a converging lens 1750, and a mirror unit 1760.
The port unit 1710 includes plural ports 1711 to 1715. The port 1711 is an input port through which an optical signal is input to the optical switch 1700. The optical signal exits the port 1711 diverging and is output to a collimating lens 1721 included in the collimating lens unit 1720. The input optical signal is a multiplexed optical signal including plural wavelengths corresponding to different channels.
The collimating lens unit 1720 includes plural collimating lenses 1721 to 1725. The collimating lens 1721 collimates the diverged optical signal output from the port 1711 and outputs the collimated optical signal to the lens system 1730. The port unit 1710 and the collimating lens unit 1720 are arranged along the Z-axis.
The lens system 1730 spatially separates channels, i.e., wavelengths, of the collimated optical signal output from the collimating lens 1721 and outputs the resulting optical signal to the dispersing element 1740. The lens system 1730 includes a concave lens 1731 and a convex lens 1732. The concave lens 1731 diverges and outputs, to the convex lens 1732, the collimated optical signal output from the collimating lens 1721. The convex lens 1732 collimates and outputs, to the dispersing element 1740, the diverged optical signal output from the concave lens 1731.
The dispersing element 1740 angularly disperses the optical signal, output from the lens system 1730, about the X-axis in different directions corresponding to wavelength while outputting the optical signals in the direction of the Y-axis to the converging lens 1750. The dispersing element 1740 includes a transmissive diffraction grating.
The converging lens (converging optical system) 1750 converges, onto corresponding mirrors included in the mirror unit 1760 and according to wavelength, the optical signals output from the dispersing element 1740. Also, according to wavelength, the optical signals pass through different positions on the converging lens 1750.
The mirror unit 1760 includes plural mirrors arrayed along the X-axis. The mirrors correspond to the different wavelengths, respectively. For example, mirrors 1761, 1762 and 1763 correspond to wavelengths λ1, λ20, and λ40, respectively.
Each of the mirrors 1761 to 1763 respectively reflects an optical signal having a wavelength corresponding thereto. The reflected optical signals are among the converged optical signals output from the converging lens 1750 and reflected toward the converging lens 1750. The mirror unit 1760 includes a control unit that controls the reflection angle of each of the mirrors by rotating each mirror about the X-axis (a first rotation axis).
The reflected optical signals pass through the converging lens 1750, the dispersing element 1740, and the lens system 1730 to be output to the collimating lens unit 1720. The collimating lenses 1722 to 1725 correspond to the reflection angles of the mirrors 1761 to 1763 of the mirror unit 1760, and are arrayed along the Z-axis. The collimating lenses 1722 to 1725 output the received optical signals to ports 1712 to 1715 included in the port unit 1710.
The ports 1712 to 1715 are output ports corresponding respectively to the collimating lenses 1722 to 1725. The ports 1712 to 1715 receive the collimated optical signals output from the collimating lenses 1722 to 1725 and output the optical signals from the optical switch 1700.
According to the above configuration, the control unit changes the reflection angle of the mirrors according to the wavelength corresponding thereto. Thus, according to each optical wavelength included in the input optical signal, the optical switch 1700 selects, from among the ports 1712 to 1715, a port to output the optical signal.
Furthermore, the optical switch 1700 slightly changes the reflection angles of the mirrors from the optimal angles for coupling the reflected optical signals to the ports 1712 to 1715 to decrease the coupling rate. Thus, the optical switch 1700 attenuates, by an arbitrary level, the optical signals output from the ports 1712 to 1715 by rotating the mirrors about the X-axis.
However, the optical switch 1700 above has a problem in that since optical signals of different wavelengths respectively pass through different positions on the converging lens 1750, an optical signal passing through a position away from the center of the converging lens 1750 is likely to be affected by aberration of the converging lens 1750. In particular, when many wavelengths are used in WDM transmission, the effective area of the converging lens 1750 becomes larger, increasing the effect of the aberration of the converging lens 1750.
FIG. 18A is a graph indicating a transmission band characteristic of the conventional optical switch. FIG. 18B is a graph indicating a short-wavelength-side transmission band characteristic of the conventional optical switch. FIG. 18C is a graph indicating a long-wavelength-side transmission band characteristic of the conventional optical switch. The horizontal axis indicates a wavelength (channel), and the vertical axis indicates a transmissivity (decibel (dB)).
A reference numeral 1801 indicates a range of wavelengths used in the WDM transmission. A reference numeral 1802 indicates a transmission band characteristic of the optical switch 1700 when an attenuation level is set to 0 dB by decreasing the coupling rate of the port unit 1710. A reference numeral 1803 indicates the transmission band characteristic when the attenuation level is set to 20 dB. An optical signal corresponding to a channel 20 passes a point closest to the center of the converging lens 1750.
As shown in the FIGS. 18A to 18C, the transmissivity becomes highest at the channel 20. The farther away a channel is from the channel 20, the lower the transmissivity becomes due to the effect of aberration, causing a slope of the transmission band characteristic for each channel. The slope becomes steeper when the attenuation level is 20 dB than when the attenuation level is 0 dB.
The slope reduces the communication band. In particular, when the output optical signal is attenuated by decreasing the coupling rate of the port unit 1710, the slope becomes steeper, causing a large reduction of the communication band. Furthermore, the use of plural compensating lenses to correct the aberration of the converging lens 1750 means increased components, resulting in reduced long-term reliability and increased element costs.